1. Field of the Invention
The present invention generally relates to nuclear power plants, and more particularly to safety mechanisms for preventing dropped control rods in such systems.
2. Background Information
In nuclear power plants, control rods are lifted and lowered within a reactor core to control the rate or reaction and the energy output from the nuclear reactor. These control rods are placed in proximity to fuel elements, which contain nuclear fissionable fuel. Generally, the greater the number of neutrons in the reactive region, the greater the number of fissions of fuel atoms that take place, and consequently the greater the amount of energy released. Energy in the form of heat is removed from the reactive region by a coolant, which flows through the region and then flows to a heat exchanger wherein the heat from the reactor coolant is used to generate steam for driving turbines to generate electricity.
To decrease the energy output of the nuclear reactor, the control rods, made of a material that absorbs neutrons, are inserted into the reactive region. The greater the number of control rods and the farther the control rods are inserted within the reactive region, the greater the number of neutrons will be absorbed. Hence, the energy output of the nuclear reactor will be decreased. Conversely, to increase the output of the nuclear reactor, the nuclear control rods are withdrawn from the reactive region. When this occurs, the number of neutrons absorbed decreases and the number of neutrons that are enabled to cause the collisions increases. This causes increased fission to take place within the core. Thus, the position and location of these rods are crucial to the correct operation of the power generating facility.
Control rods are typically arranged in banks, with each bank comprising a number of groups of control rods. The proper placement and movement of these control rods is crucial to the proper functioning of the reactor and the safety of all those around the plant.
A variety of prior art systems have been utilized to control the placement and movement of the control rods. One of these systems is a jack type system that utilizes a plurality of electrical coils to incrementally insert or withdraw each control rod within a reactor. These incremental steps are usually repeated as many times as necessary by the various banks, or groups of banks, of control rods to position the rods in a desired location to produce a desired amount of output from the reactor.
An example of such a mechanism is found in U.S. Pat. No. 3,158,766. This mechanism describes a set of three inductive coils, one for gripping, one for lifting, and one for holding the control rod in a stationary position. Therefore, when there are a number of control rods within a reactor, there will also be a corresponding number of gripping, lifting, and holding coils. These rods must be lifted in a predetermined order so that no one rod is above or below any of the other rods within the group. The current requirements for each of the coils within these groups will be the same for all of the corresponding coils in all of the lift, gripper, and holding or stationary coils. In order to control and monitor the movement of these rods, various systems have been described.
U.S. Pat. No. 3,796,890 ('890) describes such a control system. In this system, a half wave solid state rectifying bridge is provided for applying multi-level direct current from a three-phase source to a single or plurality of load elements. The direct current is provided according to a predetermined cyclical signal reference current to regulate and minimize the amount of energy used by the load elements. This system utilizes silicon rectifiers, switching devices or thyristors, each of which is connected with output from a three-phase power system to provide direct current to a respective coil. The value of the direct current from the outputs of these thyristors is determined according to the time at which each thyristor is fired. For example, if the thyristors are sequentially fired at predetermined points near the maximum positive point of each of the three phases of the three-phase power source, the resulting output will have a high average value. On the other hand, if the thyristors are fired at predetermined points shortly after the maximum negative point of each of the power phases, negative forcing will occur. This causes energy stored in the load element or elements to be returned to the power supply system. This system also provides for a variety of methods for regulating the angle at which the thyristors are fired, and for noting faults in a system based upon the input and regulation of produced digital signals.
Each of the inductive coils is controlled by the power regulating system. These coils provide magnetic flux to operate mechanical parts that effect movement of the rod. When it is desired to move a rod, a power regulating system provides multiple levels of current to the coils in a controlled sequence. The current levels are called zero, reduced, and full. When no movement of the rods is desired, the power regulating system energizes the stationary gripper at reduced current and all other coils receive zero current. Full current is applied to the coils only during movement, and then only briefly.
One of the problems with the invention described in the '890 patent is that this method and system cannot detect the failure of power current to arrive at its individual loads, such as the gripping, moving, and holding coils within a nuclear power plant. This prior art system only notices some faults, and, when it does, the device attempts to hold the rods by applying reduced current to the moving gripper. Particularly, this system is unable to detect the presence of blown fuses in a moving or stationary gripper. As a result, it is possible for a rod to be dropped from an open connector near a reactor. Even if those rods only fall a fraction of an inch, it can cause imbalance to the entire system and is a dangerous situation that must be avoided. Such a result could be catastrophic because it would substantially vary the rate of reaction within the core.
Therefore, what is needed is an improved system and method for detecting faults in an electrical power system and for alarming an individual when these faults have been noticed. What is also needed is an improved system for preventing the dropping of control rods in a nuclear power generation system. An improved method for detecting these power failures and preventing the movement of rods when such a fault is detected is also needed.
Accordingly, it is an object of the present invention to provide an improved system and method for detecting faults in an electrical power system, and for alarming an individual when these faults have been noticed. It is another object of the present invention to provide an improved system for preventing the dropping of control rods in a nuclear power generation system. Another object of the invention is to provide an improved method and system for detecting these power failures, and to prevent the movement of rods when such a fault is detected.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.